In an effort to modernize the gps constellation modifications are being made to the gps space and ground control segment these changes should be transparent to is gps 200 compliant receivers however commercial and civilian users who experience impacts are encouraged to contact the navigation center or by phone at 703 313 5900. The navigation rules are much like the rules of the road on the highway they establish a consistent way to navigate safely and avoid collisions when two boats are crossing paths are on course to meet head on or when one boat wishes to overtake another. The navigation rules cover steering rules akin to rules of the road sound signaling equipment and requirements and a section on navigation lights proper and safe on water navigation has many facets. That said for the same price i would purchase and have purchased the full navigation rules and regulations handbook us coast guard with the rules explanations and data it is a much better reference source